When can I transplant Calla Lily Plants?

I saved my calla lily bulbs from last year in the basement over the winter. It must have been warmer than I thought down there because last week I found all of them had 1 inch sprouts. I've potted them and they're growing well. When can I transplant into my garden? Normal last frost date is May15th in this zone. Is that too early? Should I wait until June 1st or later?